The invention relates generally to apparatus and methods for coating a liquid composition onto a moving substrate to form a coated layer thereon; and, more particularly, to coating apparatus and methods utilizing a backing roller while providing an electrostatic field at the dynamic wetting line where the coating liquid meets the moving substrate.
In coating a liquid composition from a coating die, hopper, or similar coating device onto a first or xe2x80x9cfrontxe2x80x9d surface of a moving web substrate, it is well known in the coating art to precisely position and support the substrate by guiding the substrate around a rotating backing roller spaced apart from the coating device. The distance between the front surface of the web and the coating device is referred to as the xe2x80x9ccoating gap.xe2x80x9d The web is thus supported directly by the surface of the backing roller through a substantial angle of rotation, or xe2x80x9cwrap,xe2x80x9d typically between about 90xc2x0 and 180xc2x0.
The front and back surfaces of the moving web carry boundary layers of air, each of which can create different problems in achieving stable coatings at high coating speeds. In the prior art, the preferred solutions to these differing front and back surface problems can be mutually incompatible.
The boundary air layer on the back surface of the web is drawn into the entrance nip formed between the web and the backing roller, which in the older prior art is typically a smooth-surface roller. At lower conveyance speeds, for example, 0.5 m/s, the air is squeezed out at the nip by tension in the web, and the web is supported without slippage on the roller. However, as conveyance speed is increased, the boundary air is incompletely squeezed out and the web begins to float on a dynamic cushion of air between the web and the roller and thus traction between the web and roller diminishes. This can lead to at least three unwanted effects: the web may wander laterally on the roller, resulting in intermittent honing of the web back surface and coating off the edge of the web; the web may not turn synchronously with the backing roller, resulting in scratching or honing of the web back surface and irregularly variable web speed at the coating point; and the coating gap may be decreased irregularly and unacceptably by the air cushion, causing unpredictable and unacceptable thickness variations in the coating.
It is well known in the prior art to relieve the back side boundary air layer by providing any of various incuse patterns in the surface of the backing roller. These patterns may include, for example, a random surface comprising lands and incuse areas which may be varied in the percentage of surface area occupied by each (see for example U.S. Pat. No. 4,426,757 to Hourticolon, et al.). More commonly, an axially central portion the roller is circumferentially scribed with a pattern of shallow grooves. See, for example, U.S. Pat. No. 3,405,855 to Daly et al. and U.S. Pat. No. 4,428,724 to Levy. Such circumferential grooves are known in the photographic coating art as xe2x80x9cmicrogroovesxe2x80x9d and may take the form either of a plurality of truly circumferential closed grooves, each in a plane orthogonal to the roller axis, or of a single continuous spiral groove of appropriately shallow pitch. The performance of these two groove patterns is substantially equivalent. A pattern commonly in use in the coating of photographic products employs 1 groove per axial millimeter (gpmm) of roller surface, each groove being 0.3-0.6 mm wide at the roller surface and about 50 to 130 xcexcm deep (see U.S. Pat. No. 6,177,141 to Billow, et al.). This pattern, provided over an axially central portion of a backing roller, can provide suitable traction and conveyance stability of a flexible plastic web substrate around a coating backing roller about 10 to 20 cm in diameter at linear speeds exceeding 5 m/s, unit area traction being substantially increased over that exhibited by a smooth roller despite the loss in roller surface area available for contact with the web.
The front surface boundary air layer can create a similar problem in engaging the coating composition as it is being applied from a hopper to the web surface. As coating speed is increased, a critical speed is encountered at which air begins to be entrained under the coating composition at the coating point, preventing the composition from wetting the web along a uniform line and thus unacceptably disrupting the uniformity of coating. It is well known in the coating art that imposing an electric field between the front surface of the web substrate and the hopper can raise significantly this critical speed for air entrainment (AE), for example, from about 2 m/s to about 6 m/s (see for example U.S. Pat. No. 4,837,045 to Nakajima). This technique is referred to as electrostatic assist for coating (ESA).
A serious problem can arise, however, in using ESA when coating onto a web supported by a grooved backing roller. A periodic coating thickness non-uniformity, referred to herein as groove lines, tends to form in the lower liquid layers as they are applied to the web, the lines being an image of the backing roller surface pattern. The electrostatic force generated on the coating composition is proportional to the square of the imposed electric field (E2). Therefore, it follows that the magnitude of coating nonuniformity is proportional to any variation in E2 occurring in the immediate vicinity of the lower surface of the coating composition as it is contacting the web. The electric field is inversely proportional to the dielectric gap between the roller surface and the front surface of the web understanding that over land areas of the roller, the gap is simply the thickness of the web, whereas in grooved areas, the gap includes the depth of the grooves. Thus there exists a pattern of periodic variation in electric field, and ESA, exerted on the coated fluid along the axial direction of the roller, creating a groove line pattern in the coating.
Multi-layer coating packs or composites having a relatively low bottom layer viscosity, for example, 4 centipoises (cP), are especially prone to formation of groove lines. As coating speed or viscosity is increased, the prevention of front-side air entrainment, even with a grooved coating backing roller, typically requires progressively higher voltages of ESA, which can, in turn, result in more intense groove lines in the coating. Thus, in the known art, grooving the backing roller to relieve the back side boundary layer problem is antithetical to increasing ESA voltage to relieve the front side boundary layer problem. The propensity to form groove lines is thus a serious impediment to achieving high coating speeds (in excess of 2 m/s) or high viscosity (in excess of 10 cP at 105 reciprocal seconds) as may be desirable for increased productivity and coated uniformity.
Another approach to relieving the back side boundary layer problem is to use a nip roller to press the web against a smooth coating backing roller and squeeze out the air entrained between the web and the roller. This nip roller would be located prior to the coating application point. The use of a smooth backing roller would avoid creation of non-uniform ESA. However, this nip roller would need to contact the face side of the web immediately prior to coating. In many situations, it is desirable to avoid contact with the face side of the web until the last layer of coating has been applied and sufficiently dried. In addition, the use of a nip roller increases the chances of causing creasing, particularly with thinner webs.
In prior art practice, using a prior art backing roller having a pitch of 1 gpmm and a groove depth of 130 xcexcm and a groove width of 500 xcexcm, for a given web substrate having a given thickness and being coated at a given web speed, the level of ESA is adjusted until a very low but acceptable intensity of groove lines is achieved. Typically, the coating speed and the ESA level are co-optimized to achieve the maximum possible coating speed with the highest possible ESA voltage, which coating speed may be substantially less than that permitted solely by the traction afforded by the grooves. Coating speeds higher than this may be used only at a sacrifice in coating uniformity.
Thus there is a need for an improved coating apparatus and method which provides suitable web traction at high coating speeds (greater than 2 m/s) while simultaneously allowing high levels of ESA (greater than the ESA level provided by applying 300V of voltage differential between the surface of a coating backing roller and the application hopper) without causing unacceptable groove lines in coatings.
It is therefore an object of the present invention to provide an improved backing roller which permits stable coatings of acceptable thickness uniformity to be made at high coating speeds in the presence of high levels of ESA.
It is a further object of the present invention to provide a method for preventing unacceptable levels of groove line non-uniformity when using ESA in the presence of a grooved backing roller.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing a coating apparatus with a backing roller having a significantly higher spatial frequency of circumferential grooves, preferably at least about 2 grooves per millimeter (gpmm), than that of prior art backing rollers having 1 groove per axial millimeter of roller surface. Preferably, the grooves are significantly shallower than prior art grooves (a depth of about 75 to 150 xcexcm), and most preferably having a depth of about 45 xcexcm. The finer, shallower groove pattern reduces axial spatial variations in ESA force by as much as a factor of 10 or more by decreasing the axial distance between lands and by decreasing the depth of the grooves. It is believed that such axial spatial variations along the roller surface give rise to an irregular or scalloped dynamic wetting line where the liquid composition meets the web surface; and further, that the magnitude of groove line non-uniformity is directly proportional to the magnitude of deflection of the wetting line, and further, that the magnitude of deflection is directly proportional to the square of the wavelength of the deflection. Thus, increasing the groove frequency, or xe2x80x9cpitch,xe2x80x9d by a factor of two (from 1 to at least 2 gpmm) can reduce the magnitude of groove line non-uniformity by a factor of at least 4. In a preferred embodiment, a backing roller has a groove pitch of 4 gpmm, a groove depth of 45 xcexcm, and a groove width of 200 xcexcm, providing a non-uniformity reduction of about 160xc3x97 over a prior art roller having a pitch of 1 gpmm, a groove depth of 130 xcexcm, and a groove width of 500 xcexcm. Furthermore, the grooved pattern of the preferred embodiment extends across the axial length of the backing roller so as to completely underlie the full width of the coating composition.
By modifying the groove depth, the conveyance performance of the backing roller with finer groove patterns is comparable to that of backing rollers with prior art groove patterns. At linear speeds up to at least 7.5 m/s, with web tension at about 0.75 pounds-force per lateral inch of web, a 10 cm diameter backing roller having a groove frequency of 4 gpmm, a groove depth of about 45 xcexcm, and a groove width of about 200 xcexcm, has been found to provide conveyance performance substantially the same as that of a 1 gpmm roller having a groove depth of about 130 xcexcm and a groove width of about 500 xcexcm.
In the practice of the method of the present invention, a groove pitch and depth are provided in a backing roller which reduces the intensity of groove lines in the coating to an acceptable level and provides adequate conveyance performance, and then a level of ESA is determined empirically which prevents air entrainment of a given composition when coated onto a web of given thickness at a desired coating speed. This permits either coating at a higher speed or higher viscosity than may be achieved using the above-described prior art method with a prior art backing roller, or greatly reduced groove line nonuniformity at a given coating speed and viscosity.
It should be appreciated by those skilled in the art that the magnitude of groove line nonuniformity that is acceptable depends on many factors, including the type of product being manufactured, and photographic products have a relatively low tolerance for groove line nonuniformity. Even within the field of photographic products, the acceptable magnitude can vary by more than ten fold, where products that are magnified greatly or products with a relatively high contrast have the tightest tolerances.